Some medical procedures require a needle or needle-like instrument to be inserted into a patient's body to reach a target. Examples of these procedures include tissue biopsies, drug delivery, drainage of fluids, ablation for cancer treatment, and catheterization. Some of these procedures can be done manually without any additional guidance other than the sense of feel and visualization of the surface of the body. Other procedures are difficult to perform without additional guidance because the target is deep, the target is small, sense of feel is inadequate for recognizing when the needle's tip has reached the target, or there is a lack of visual landmarks on the body surface. In those cases, providing the doctor with an image of the interior of the body in the vicinity of the target could be beneficial. It would be particularly beneficial to provide real-time images of both the target and the needle as it progresses towards the target.
A particularly challenging needle insertion procedure is required in epidural anaesthesia, often referred to as an “epidural” in the field of obstetrics. Epidural anaesthesia is administered in the majority (>80% of women in labour) of patients for pain relief of labour and delivery in North American hospitals. Epidural anaesthesia involves the insertion of a needle into the epidural space in the spine. The anatomy of the back and spine, in order of increasing depth from the skin, includes the skin and fat layers, a supraspinous and interspinous ligament, the epidural space, the dura mater and spinal cord. A doctor must insert the needle through these layers in order to reach the epidural space without over-inserting the needle and puncturing the thin dura mater surrounding the spinal cord.
The traditional procedure of epidural needle insertion may be as follows. The patient is seated with the doctor facing the patient's back. The doctor chooses a puncture site between the vertebrae based on feeling the protruding spinal processes. After choosing an insertion point on the body, the doctor typically inserts the needle in a plane midline with the long axis of the spine. A saline-filled syringe is attached to the needle so the doctor can apply pressure to the plunger of the syringe, as the needle in incrementally advanced toward the epidural space, and feel how easily saline is injected into the tissue. This is called the “loss-of-resistance” method because resistance falls when the needle tip enters the epidural space. In this way, the sense of feel is the main method for determining when the needle tip has reached the epidural space because the saline is easily injected into the epidural space compared to the tissue encountered before the epidural space. This method can result in failure rates of 6 to 20% depending on the experience and training of the doctor. Complications include inadvertent dura puncture resulting in loss of cerebral spinal fluid and headache, as well as nerve injury, paralysis and even death. Image guidance during needle insertion would improve the accuracy of needle insertion by providing better feedback to the doctor of where the needle is located with respect to the anatomical structures including the target.
In the past several years, ultrasound has been explored as a means to provide a pre-puncture estimate of the depth of the epidural space to correctly place the needle tip. This entails an ultrasound scan prior to needle insertion so that the doctor uses the knowledge of how deep to expect the epidural space when inserting the needle. This use of pre-puncture ultrasound at the planning stage for epidural guidance has received wide interest from the anaesthesia community. It is called pre-puncture ultrasound scanning because the ultrasound is used before, but not during, needle insertion. The National Institute for Health and Clinical Excellence (NICE) has recently issued full guidance to the NHS in England, Wales, Scotland and Northern Ireland on ultrasound-guided catheterization of the epidural space (January 2008). While pre-puncture scanning is a useful advance, doctors still face challenges associated with performing needle insertion procedures without information provided by real-time imaging.
Another similar needle insertion procedure is a lumbar puncture, where a needle is inserted through both the ligamentum flavum and the dura mater into the subarachnoid space to collect cerebrospinal fluid (CSF) for diagnostic and sometimes for therapeutic purposes. Failure to penetrate the subarachnoid space with the spinal needle may require the need for fluoroscopy-guided lumbar puncture to achieve correct localization of the needle.
There have been a small number of published reports describing real-time ultrasound imaging for needle insertion procedures. However, none of these approaches have proven to be entirely satisfactory. Problems include overly limiting views of the images of the target and needle due to poor reflection of ultrasound waves, and/or inherent limitations in the ultrasound equipment. Holding an ultrasound probe in one hand, and advancing a needle into the body with the other hand leaves no hands free to attach a syringe to the needle and press the plunger to detect a loss of resistance. A conventional needle guide can be attached to the ultrasound probe to hold the needle in place, but needle guides with closed channels do not allow for easy removal of the needle from the needle guide when the tip has reached the sensitive target in the spine. Conventional needle guides mounted to an ultrasound probe are typically used with the probe pointing directly to the target (i.e. with the probe face perpendicular to the body surface) and the needle inserted at a non-perpendicular angle to the body surface. Such operation of conventional needle guides is typical for standard 2D ultrasound probes because the needle must fall within the 2D imaging plane yet the probe is directly above the target; the needle must puncture the body surface to the side of the probe and proceed toward the target at an angle of approximately 20 to 60 degrees to the body surface.